A wire EDM is defined as equipment for machining a work made of a conductive material, particularly of metal, based on the machining process, wherein: (a) an intermittent electric spark discharge is generated in a machining liquid ejected into a gap between a workpiece of a conductive material, e.g., a metal and a wire electrode of a conductive material, e.g., brass, the wire electrode being extended under tension to penetrate the work; (b) the intermittent electric spark discharge causes an extremely small region of the workpiece adjacent to the aforementioned gap (hereinafter referred to as a machining region), to be softened and/or melted; (c) the intermittent electric spark discharge incidentally causes the internal pressure of the machining region to fluctuate; and (d) the fluctuated internal pressure causes the softened and/or melted region of the conductive material, e.g., a metal, to be disintegrated into a number of small particles and removed from the workpiece.
The aforementioned wire EDM is provided with a numerical control (NC) system, or the like, to accurately regulate, with an accuracy level of 0.001 mm, the geometrical position of the workpiece relative to the supporting point of the wire electrode which has an approximate diameter of 0.03-0.4 mm, thereby accurately regulating the geometrical position of the intermittent electric spark discharge (which is identical to the location of the wire electrode in the vicinity of an electrical discharge machining plane which is defined as a plane along which electric discharge machining occurs along the wire electrode), and maintaining the gap between the wire electrode and the electrical discharge machining plane at approximately 0.03 mm. The wire EDM is further provided with a means for stabilizing the intermittent electric spark discharge. A combination of these means is effective to accurately regulate the geometrical position of the electrical discharge machining plane, resultantly realizing an excellent grade quality of machining accuracy.
Parameters allowed for regulating the wire EDM are limited to two items, including the geometrical position of the supporting point of a wire electrode with respect to a work and the aforementioned intermittent electric spark discharge. Therefore, it is essential for the wire EDM to extend the wire electrode without a sag and to maintain the gap between the wire electrode and the electrical discharge machining plane at a constant value, because it is otherwise impossible for the wire EDM to accurately regulate the geometrical position of the electrical discharge machining plane by regulating the aforementioned two parameters. Accordingly, the machining accuracy of a wire EDM greatly depends on the linearity of the wire electrode and the accuracy of the electrical discharge gap.
Thus, various efforts are being used to improve the linearity of the wire electrode and to stabilize the intermittent electric discharge to maintain the electric discharge gap at an accurately constant value.
Firstly, a V-groove-type wire guide is available in the prior art to improve the linearity of the wire electrode. As is illustrated in FIG. 1, a wire electrode 1 is extended under tension between a pair of V-groove type wire guides 2, 2', which are driven in a plane perpendicular to the wire electrode 1 by a servo mechanism. This V-groove type wire guide advantageously allows easy setting of the wire electrode and does not require any particular attention to be paid to irregularity in the diameter of wire electrode, e.g., the irregularity caused by projections, or the like, produced on the surface of the wire electrode caused by electric discharge. Incidentally, however, since the wire electrode has an approximate diameter of 0.03-0.4 mm, a large amount of tension cannot be applied thereto. In addition, a wire is hard and is inclined to stay in a bent or kinked shape, once it is bent. Due to these parameters, it is not easy to realize a satisfactory quality of linearity with a V-groove-type wire guide. In other words, there is a tendency for the deviation shown by 11 in FIG. 2 to occur. In addition, the value of this deviation is unstable or changeable and often appears with vibration.
Secondly, a wire EDM can be employed in two independent machining modes. The first is an ordinary machining mode or a non-taper machining mode in which a wire electrode is maintained in a direction perpendicular to the direction in which the wire electrode is moved with respect to the workpiece. The second one is a taper machining mode in which a wire electrode can be inclined against the plane in which the wire electrode is moved. In this taper machining mode, a V-groove-type wire guide cannot be employed due to the following reasons.
The first reason is that, since the taper machining causes each of the upper and lower guides to be driven independently of the other in the plane perpendicular to the wire electrode, the supporting point of the wire slides along the V-groove of the V-groove type-wire guide. This leaves the supporting position of wire electrode in an unstable position. This means that the supporting point is not maintained at a fixed point, readily resulting in lesser quality of machining accuracy.
The second reason is that a V-groove-type wire guide imposes a limitation on the direction in which the wire electrode is inclined. In other words, a wire electrode is not allowed to be inclined in arbitrary directions.
Accordingly, a pair of die-type wire guides 3, 3' illustrated in FIG. 3 is employed in the taper machining mode. Since a die-type wire guide imposes no limitation regarding the direction in which the wire electrode is allowed to be inclined, this wire guide can be employed in the taper machining mode. It is supposed that a die-type wire guide having a sufficiently small die diameter can maintain the distance between the supporting points of the upper and lower guides at a constant value, and that the geometrical position of the wire electrode can accurately be regulated in the region close to the electrical discharge machining plane by regulating the supporting points of the wire electrode, even if one of the upper and lower wire guides is driven independently of the other. In reality, however, a margin is required for the die diameter, for the purpose of setting a wire electrode to a wire EDM, and irregularity in the diameter caused by electric discharge must be taken into consideration.
Due to these parameters, the accuracy in regulating the position of the wire electrode with respect to the workpiece is worse for the die-type wire guides than for the V-groove-type wire guides. Although the die type wire guide supports a wire at only one supporting point, the wire is not allowed to be bent with a sharp angle. This could be a reason for the poor accuracy of positioning the wire. A combination of these parameters causes an unsatisfactory quality of machining accuracy.
Thirdly, an apparatus for guiding a wire electrode provided with a combination of a pair of V-groove-type wire guides 2,2' and a pair of die-type wire guides 3, 3' which is illustrated in FIG. 4, was developed by the present inventors for the purpose of removing the foregoing drawbacks, and a patent application was filed in the Japanese Patent Office by FANUC LTD on Apr. 27, 1982 (Toku Gan Show No. 57-70973). It was experimentally demonstrated that this apparatus could realize a high quality of machining accuracy in the taper machining mode, because it is effective in straightening the wire electrode and that this apparatus could realize a high quality of accuracy also in the non-taper machining mode. Incidentally, however, the problem remained unsolved regarding how best to assemble the various components of the apparatus into one convenient practical unit.
Fourthly, for the purpose of maintaining an intermittent electric spark discharge in a stable manner, the wire electrode is required to be surrounded by a film of a machining liquid. To satisfy this requirement, a nozzle which is illustrated by 4 in FIG. 5 and which has an approximate nozzle diameter range of 3-6 mm, is employed to allow the wire electrode to penertate it and to allow the machining liquid to flow in the direction shown in arrow A and in the shape of a liquid column surrounding the wire electrode.
FIGS. 6 and 7 illustrate two independent types of apparatuses for guiding a wire electrode of a wire EDM available in the prior art under the aforementioned technical environments.
FIG. 6 illustrates an apparatus for guiding a wire electrode of a wire EDM provided with a supporting plate 5 and a functional member 6 which is fixed at the lower surface of the supporting plate 5 and which is provided with a V-groove-type wire guide 2 and a machining liquid nozzle 4'. This apparatus is not suitable to be employed in the taper machining mode but is suitable to be employed in the ordinary or non-taper machining mode.
In contrast, FIG. 7 illustrates an apparatus for guiding a wire guide of a wire EDM provided with a supporting plate 5 and a functional member 7 which is fixed at the lower surface of the supporting plate 5 and which is provided with a die-type wire guide 3 and a machining liquid nozzzle 4". This is suitable to be employed in the taper machining mode.
The above description indicates that apparatuses for guiding a wire electrode of a wire EDM available in the prior art are limited to either one which comprises a V-groove-type wire guide and is preferably employed in the non-taper machining mode or one which comprises a die-type wire guide and is employed in the taper and non-taper machining modes. This means that an apparatus for guiding a wire electrode of a wire EDM which is adaptable for use with a device having a V-groove-type wire guide and a die-type wire guide and a device using a V-groove-type wire guide alone, is not available in the prior art.